The present invention relates, in general, to measurement while drilling (MWD) methods and apparatus, and more particularly to methods and apparatus for relative drilling direction measurement using a drill stem carrying a rotating magnet drill head.
A typical drill stem, of the type which may be used in drilling boreholes such as wells for oil or gas exploration or production, or boreholes for the installation of cables and pipelines, in addition to many other purposes, carries at its lower end a drill head which includes a motor-driven rotary drill bit. Such drill bits are mounted on an angled drill bit shaft, or bent sub, which is driven by a motor under the control of a drilling operator at the earth""s surface. The longitudinal axis of the bent sub is typically set at a small angle; for example, three-fourths of a degree, with respect to the axis of the drive motor and drill stem to allow directional drilling. The drive motor is typically mounted to the lower end of, and is coaxial with, a nonmagnetic section of the drill stem in which well survey electronics are located for measurement of well direction and location during drilling. Such drills are typically operated in one of two modes; a sliding mode or a rotary mode. In the sliding mode, the drill motor is activated to cause the drill bit to rotate while the rotational angle of the drill stem is held steady, and thus does not rotate. Since the axis of the bent sub, or drill bit shaft, is at a slight angle with respect to the axis of the drill stem and the drill motor, rotation of the bit causes the borehole to be drilled at the angle of, and in the direction of, the angle of the bent sub with respect to the drill stem axis, and this causes the borehole to change direction. The direction of the bent sub is controlled by the angular position of the drill stem and manifests itself by changing the down hole drilling direction by a small amount.
In the rotary mode of drilling, the drill stem is rotated as the down hole motor is powered to give the drill bit a compound rotation i.e. a component due to drill stem rotation and a component from the motor. This produces a continuous precession of the bent sub around the axis of the drill stem and causes the borehole to be drilled with a slight helicity. The average drilling is in the direction of the drill stem axis and with the diameter of the borehole being slightly larger than the diameter of the drill stem.
To achieve accurate directional control of the drilling, the drill operator needs to know with precision the borehole curvature being achieved. However, the measurement while drilling (MWD) equipment typically is located in a drill stem section above the drive motor, about 15 meters behind the drill bit. This means that if standard MWD equipment is relied on for the measurement of the borehole inclination and azimuth, the drill system will have advanced 15 meters before any measurement of a change in borehole direction can be obtained. In many applications, such as in the drilling of intersecting wells or in the drilling of closely spaced parallel wells such as those used in steam assisted gravity drainage (SAGD) wells, where parallel wells are spaced, for example, by approximately 5 meters over a kilometer of length, the problems of accurate directional measurement and drilling control are recurring and very serious.
In current practice, there are several systems used for overcoming this delay in the measurement of borehole inclination, but none for borehole azimuth. Typically, accelerometers and transmitters are located at or near the drill bit, and these transmit data past the motor to the MWD equipment, using either acoustic or electromagnetic transmission signals. However, these systems have serious problems, since such communication links are unreliable and the drilling must be stopped to measure the drill bit inclination. Such stoppages not only delay the drilling, but can result in the drill stem sticking in the borehole.
The current practice of drilling steam assisted gravity drainage (SAGD) well pairs is based upon a system disclosed in U.S. Pat. No. 5,485,089 and IADC/SPE paper 27466 which allows precise location determination of the MWD sensor package relative to a reference point approximately opposite the MWD package in a reference well. However, this method gives no information about the current relative drilling direction, i.e., whether the current drilling path is parallel to the reference well. It is only after drilling has proceeded to far beyond the point where the current measurement is being made that this evaluation can be carried out.
U.S. Pat. No. 5,589,775, which discloses a method of utilizing a drill bit with a rotating magnet to measure the azimuthal direction to an adjacent parallel wellbore, complements the present invention for obtaining a relative direction determination to a remote point. However, to produce parallel well pairs, knowledge of the current relative drilling direction relative to the direction of the reference well is very important, and there is a serious need to do this better than has previously been possible.
Similar concerns arise when it is necessary to drill precisely to a predetermined, distant point. Present practice is based upon determining the coordinates of the present drill bit location and those of the target and adjusting the drilling as it proceeds. Until recently these coordinates were determined by integrating a large ensemble of survey measurements from a surface location to the drill bit in conjunction with land surveys, with an ensemble of survey measurements to the target location. Recent developments have focused on making in situ determination of the apparent target location relative to the current drill bit location (A. G. Nekut, A. F. Kuckes and R. P. Pitzer, Rotating Magnet Rangingxe2x80x94a new drilling guidance technology, 8th One Day Conference on Horizontal Well Technology, Canadian Sections SPE/Petroleum Society, Nov. 7, 2001).
U.S. Pat. No. 5,258,755 discloses a method for determining relative drilling direction utilizing a drill bit with a rotating magnet in conjunction with an axial electromagnet as part of the drilling assembly. While the physical principles of the method are sound, the encumbrance associated with incorporating an electromagnet into a drilling assembly has inhibited its development.
The present invention overcomes the problems of previous approaches to directional drilling control by providing an in situ determination of relative direction from a current drilling direction to a target.
One embodiment of the present invention is directed toward a method for measuring borehole curvature near a drill bit by measuring relative borehole direction at the drill bit during drilling of a borehole with respect to the direction of the axis of an MWD sensor package usually located approximately 15 meters behind the drill bit. An MWD package typically includes magnetometers for measuring the three vector components and includes inclinometers for measuring the three vector components of the earth""s gravity. The measurement of direction is accomplished by mounting a permanent magnet on the drill bit for rotation, with the magnetic axis of the magnet lying in a plane perpendicular to the axis of rotation of the drill bit, and by providing alternating magnetic field sensors in a nonmagnetic section of the drill stem above the drive motor. These A.C. sensors, which may be incorporated into the conventional MWD equipment, detect and measure the x, y and z vector components of the alternating magnetic field produced by the rotating permanent magnet when the bit is driven by the bit motor. If the shaft connecting the drill bit to the motor is straight, i.e., coaxial with the axis of the motor and drill stem, the permanent magnet will be in a plane perpendicular to the axis of the drill stem, and would produce a uniform magnetic field in an x y plane perpendicular to the axis of the drill stem at the location of the sensor magnetometer. There would be no z axis field in this situation. It will be understood that since the drill bit is typically carried on a shaft emanating from a bent motor housing, as discussed above, the initial field produced by the permanent magnet normally is at a known angle with respect to the lower end of the motor by the amount of the bent motor housing angle. In accordance with this invention, this initial field is readily subtracted from measured values, so that the xe2x80x9ceffectivexe2x80x9d z axis field component is zero for some discussion purposes.
In the rotating mode of operation, the drill bit is rotated by the downhole motor, to which is attached the permanent magnet, so that the drill bit and motor rotate with respect to the drill stem and the MWD package to which they are attached, and the drill stem is also rotated. This causes the borehole to be drilled in the axial direction of the drill stem. In the sliding mode, the drill stem orientation is held fixed and the drill bit is rotated, causing the drill to advance at an angle with respect to the axis of the drill stem to cause the borehole to change direction. The sliding mode produces borehole curvature and produces a bend in the drive motor and drill stem, causing the angle of the plane of the permanent magnet to change with respect to the axis of the MWD sensor in the drill stem. This change in magnetic field direction produces a z vector component in the rotating magnetic field at the sensor, and this z component is a measure of the amount of the bend and thus of the change in the borehole direction.
In use, therefore, when a driller starts a drilling operation in the sliding mode, the drill stem, the motor and the drive shaft will start to bend as the bit takes hold. Typically, this produces a change in the direction of the borehole of anywhere from approximately 3 to 15 degrees after 30 meters of drilling, and this causes the direction of the axis of rotation of the permanent magnet to shift with respect to the axis of the drill stem at the MWD sensor equipment. This change can be accurately measured by the amplitude of the alternating z component of the magnetic field at the MWD sensors, and provides a direct measurement of changes in the borehole direction. This bending can be measured with great sensitivity, because the z component is essentially a null measurement. It is self-calibrating, since when there is no bending, there is effectively no signal. When there is a bend, it is only necessary to measure the amplitude and phase of the z component of the field relative to the x and y components to get a direct measurement of the magnitude and direction of the borehole bend relative to the sensing magnetometers.
Since the rotating magnet generates a rotating field which manifests itself as alternating magnetic field components at the sensors, phase averaging and phase locked loops can been used to extract very small signals. This overcomes the errors which might be caused by the intrinsic vibration of the drill stem during drilling and the consequent interaction of the magnetometers with the earth""s magnetic field. Tests indicate that the effect of background fields does not prevent accurate measurement of inclination and direction in accordance with the present invention.
In a second embodiment of the invention, the sensors are mounted in an existing borehole for measurement of relative bit inclination and azimuth in an adjacent borehole as it is being drilled. For example, in the case where parallel wells are to be drilled, as in SAGD drilling, the drill operator needs to know with great accuracy the relative direction of the well being drilled with respect to an existing well with a casing. It is not only necessary to know that the separation between the wells is within tolerances, but just as important to the driller is knowing whether the current direction of drilling with respect to the existing well is correct, for if the direction is not known, there is a risk that the required separation will be lost. The separation is determined by measuring the direction of drilling as the drilling progresses and mathematically modeling the direction of the drilled well with respect to the reference well. In accordance with this embodiment, parallel drilling is accomplished by placing a stationary sensor, in an existing, or reference well. A permanent magnet is mounted on the drill bit in the well being drilled and as the drill bit passes the stationary sensor, the magnetic field component in the z direction can be used to ascertain the degree to which the new borehole is converging or diverging with respect to the reference well and also the skewness angle of the two wells.
In a third embodiment of the invention the method is applied to the problem of drilling guidance for a borehole which is to precisely intersect a distant point target. At the outset, where the method becomes an operable, the target and current depth of drilling locations may have an uncertainty in their relative locations of 10 meters or more if the points are far from their surface entry points. In this case a rotating magnet is again fixed to the drill bit together with a standard MWD orientation package in the drilling assembly, as described above. An instrument package, which includes 3-component alternating magnetic field sensors and orientation sensors using for example the Earth""s magnetic field and Gravity direction together with means for transmitting data to the surface is placed at or near a target point. The amplitude and phase of the alternating magnetic field component for the current direction of drilling, relative to the alternating magnetic field components perpendicular to this direction, are used to determine the direction of drilling relative to that of a straight line connecting the target location and the drill bit location.